We propose to study patients with antibiotic-induced colitis and other gastrointestinal side effects of antibiotics to define any microbiological, immunological or toxic mechanisms responsible for these processes. We will attempt to prevent or treat these conditions with various antimicrobial drugs or other substances found promising on the basis of these studies. We will study antibiotic-induced colitis in hamsters and other laboratory anlimals, examining in these models the same hypotheses as are being tested in humans, and with similar goals. We will attempt to extend these studies to other inflammatory bowel diseases of humans, such as ulcerative colitis and regional ileitis.